The spike-triggered averaging technique will be used to analyze functional synaptic connections between Ia afferent fibers and alpha-motoneurons in isolated spinal cord segments. We are interested in studying mechanisms which are responsible for increased EPSP amplitudes which have been observed soon after spinal cord transection. We will test the possibility that a humoral mechanism is involved and also the possible role of ephaptic interaction at the cut end. Increases in EPSP amplitude in chronic spinal animals seem to require extensive denervation of motoneurons and we will attempt to demonstrate whether sprouting of Ia fiber terminals has occurred. The ability of other afferent systems impinging on motoneurons to undergo such changes will be studied. The possible role of transection of the dorsal column branch of the Ia fiber in causing such alterations will be explored. We will attempt to show whether presynaptic disinhibition of Ia fibers plays any role in their increased synaptic action. The finding that Ia fibers project to more motoneurons in the isolated spinal cord will be examined in the context of seeing whether such changes are independent of the increased EPSP amplitudes which also occur. Spinal cats will be subjected to treadmill training to see whether this influences the changes in Ia fiber-motoneuron synapses. Finally, motoneurons will be studied histologically using HRP injected through the recording microelectrode to see to what extent the physiological changes can be accounted for by morphological ones.